This invention relates generally to air separation and more specifically to cryogenic air separation and nitrogen and/or oxygen production.
Frequently, in a column system for air separation, there is a need to introduce low-pressure nitrogen liquid to a high pressure column as reflux. Examples of column systems where this occurs include:
Side-by-side column arrangements for the production of nitrogen and/or oxygen;
Column systems for the production of nitrogen and/or oxygen with dual reboiler and nitrogen expansion; and
Column systems for the production of high pressure nitrogen with nitrogen liquid reflux pumped from the low pressure column to the high pressure column.
These column arrangements are described in detail below.
In a typical air separation unit, for example the configuration shown in FIG. 1, there are at least two distillation columns: a high pressure column 110, and a low pressure column 120. These columns are heat integrated through reboiler-condenser 130 and the low pressure column is usually built on top of the high pressure column.
With the increasing trend toward higher efficiency distillation and higher purity of products, the height of the distillation column in such a configuration increases. The height of the combined high pressure-low pressure column system ultimately becomes so tall that the design of the entire system is prohibitively expensive. Stacking the columns is also not typically desired for larger plants, where the diameters of the columns are large and the columns are heavy.
To avoid these problems, conventional high pressure and low pressure columns can be built side-by-side. The reboiler-condenser can be located on top of the high pressure column (such as the configuration shown in FIG. 2) or in the bottom of the low pressure column (shown in FIG. 3). In both of these cases a pump is necessary. According to U.S. Pat. No. 6,148,637, and as shown in FIG. 2, liquid oxygen in stream 240 is pumped, using pump 250, from the bottom of low pressure column 220 to reboiler 230 located on top of high pressure column 210.
U.S. Pat. No. 6,148,637 discloses a three component system, comprised of a lower pressure column, a higher pressure column, and a heat exchanger. Included in this system is a pump for transporting liquid from the bottom of the lower pressure column to a vaporizer-condenser at the top of the higher pressure column.
As illustrated in FIG. 3, nitrogen liquid in stream 360 is pumped, using pump 350, from reboiler-condenser 330 located in the bottom of low pressure column 320 back to the top of high pressure column 310 as reflux. Usually two pumps instead of one are installed for the same servicexe2x80x94a working pump and an idle nitrogen liquid pump that serves as a spare. Cryogenic liquid pumps are expensive, require periodic maintenance and, because they contain moving parts, are more likely to fail than stationary equipment.
A column system for the production of nitrogen and/or oxygen with a dual reboiler and nitrogen expansion has been described in U.S. Pat. No. 4,796,431, and is shown in FIG. 4. In this arrangement, air is introduced to high pressure column 410 and product nitrogen and oxygen are withdrawn from low pressure column 420. The columns are heat integrated by lower reboiler-condenser 430 and upper reboiler-condenser 440. Nitrogen in stream 432 from the top of high pressure column 410 is divided into streams 434 and 436. Stream 436 is expanded in expander 438, thus creating necessary refrigeration. The output of expander 438 is then condensed in upper reboiler-condenser 440 and the resulting stream 442 is divided into streams 444 and 446. Stream 444 is then fed to the top of the low pressure column 420 as reflux. Stream 446 is directed to the high pressure column as additional reflux. Because its pressure was reduced by prior expansion, however, stream 446 needs to be pumped using pump 450.
A column system for the production of high pressure nitrogen with nitrogen liquid reflux pumped from the low pressure column to the high pressure column has been described in U.S. Pat. No. 5,098,457, and it is shown in FIG. 5. In this arrangement, air is introduced to high pressure column 510 and high pressure nitrogen product is withdrawn from the top of this column as stream 515. High pressure column 510 is heat integrated with low pressure column 520 through reboiler-condenser 530. Nitrogen overhead from the top of low pressure column 520 is condensed in condenser 540 and a part of this condensate in stream 545 is pumped using pump 550 back to high pressure column 510 as additional reflux 560, thus increasing the recovery of high pressure nitrogen in stream 515.
A goal of the present invention is to introduce low pressure nitrogen reflux into a distillation column that operates at a higher pressure without using pumps. More specifically, the present invention includes a method of separating air to produce nitrogen and/or oxygen in a system having at least one high pressure column, at least one low pressure column, and a reboiler-condenser, the method comprising the steps of generating a high pressure nitrogen stream from the high pressure column, and using energy from the high pressure nitrogen stream to provide nitrogen reflux to the high pressure column.
The method according to the present invention, more specifically, comprises generating a high pressure nitrogen vapor stream and condensing a portion of the high pressure nitrogen vapor stream to form a high pressure nitrogen liquid stream which is reduced in pressure by transferring it to a reflux vessel, where it is collected, then using a portion of the high pressure vapor stream not condensed to pressurize the reflux vessel to a pressure equal to the high pressure column and cause the nitrogen liquid collected therein to flow into the high pressure column under force of pressurization coupled with the static head of the nitrogen liquid. In an alternative embodiment, the high pressure nitrogen vapor stream may optionally be expanded prior to condensing it to form the low pressure nitrogen liquid stream. This stream, after expansion and condensing, may then be further reduced in pressure and transferred to the reflux vessel.
In an alternate embodiment, the method of the present invention comprises generating a high pressure nitrogen vapor stream, dividing the high pressure nitrogen vapor stream into two streams, one of which is condensed to form a nitrogen liquid stream, and the other of which is combined with the thus formed nitrogen liquid stream to form a two-phase mixture which is passed to a raised reflux vessel wherein the nitrogen liquid is collected and refluxed back to the high pressure column.
The present invention also provides a method of separating air to produce nitrogen and/or oxygen in a system having at least one high pressure column, at least one low pressure column, and a reboiler-condenser located in the bottom of the low pressure column, comprising the steps of generating a first high pressure nitrogen vapor stream and a second high pressure nitrogen vapor stream from the high pressure column, condensing the first high pressure nitrogen stream to form a high pressure nitrogen liquid stream, dividing the high pressure nitrogen liquid stream into a low pressure column liquid reflux stream and a high pressure column liquid reflux stream, and using the second high pressure nitrogen vapor stream to supply energy to cause the passage of the high pressure column liquid reflux stream to the high pressure column.
Another embodiment of the present invention comprises the steps of generating a high pressure nitrogen vapor stream from the high pressure column, dividing the high pressure nitrogen vapor stream from the generating step into a first high pressure nitrogen vapor stream and a second high pressure nitrogen vapor stream, condensing the first high pressure nitrogen vapor stream in a reboiler-condenser at the bottom of the low pressure column to form a high pressure nitrogen liquid stream, dividing the high pressure nitrogen liquid stream into a low pressure column liquid reflux stream and a high pressure column liquid reflux stream, and using the second high pressure nitrogen vapor stream to supply energy to cause the passage of the high pressure column liquid reflux stream to the high pressure column.
Still another embodiment comprises a method of separating air to produce nitrogen and/or oxygen in a system having at least one high pressure column, at least one low pressure column, and a condenser, the method comprising the steps of withdrawing a high pressure nitrogen vapor stream from the high pressure column, withdrawing a low pressure nitrogen vapor stream from the low pressure column, condensing the low pressure nitrogen vapor stream to form a low pressure nitrogen liquid stream, transferring the low pressure nitrogen liquid stream to a reflux vessel that is at a transfer pressure less than the pressure of the high pressure column, and passing a portion of the high pressure nitrogen vapor stream to the reflux vessel to increase the pressure within the reflux vessel to a pressure equal to the high pressure column whereby the nitrogen liquid in the reflux vessel is passed to the high pressure column.
Yet another embodiment of the present invention is a system for separating air to produce nitrogen and/or oxygen comprising a high pressure column for producing a first high pressure nitrogen vapor stream and a second high pressure nitrogen vapor stream, a low pressure column for producing a low pressure nitrogen vapor product stream, a condenser to receive the first high pressure nitrogen stream to form a high pressure nitrogen liquid stream, and at least two reflux vessels in fluid communication with the high pressure column for receiving the high pressure nitrogen liquid stream from the condenser, wherein the second high pressure nitrogen vapor stream is in fluid communication with the at least two reflux vessels to provide pressurization of the vessels.
Still yet another embodiment is a system for separating air to produce nitrogen and/or oxygen comprising a high pressure column for producing a first high pressure nitrogen vapor stream, a low pressure column for producing a low pressure nitrogen vapor product stream, a condenser to receive the first high pressure nitrogen stream to form a high pressure nitrogen liquid stream, and a reflux vessel in fluid communication with the high pressure column for receiving the high pressure nitrogen liquid stream from the condenser, wherein the reflux vessel is disposed above the high pressure column at a height sufficient to generate a static head pressure necessary to deliver the high pressure nitrogen liquid stream from the reflux vessel to the top of the high pressure column.
Yet still another embodiment of the present invention is a system for separating air to produce nitrogen and/or oxygen, comprising a high pressure column for producing a high pressure nitrogen vapor stream, a low pressure column for producing a low pressure nitrogen vapor product stream, a condenser to condense the low pressure nitrogen vapor product stream, and a reflux vessel in fluid communication with both columns for receiving the low pressure nitrogen liquid stream from the low pressure column and the high pressure nitrogen vapor stream from the high pressure column, wherein the reflux vessel is disposed above the high pressure column at a height sufficient to generate a static head pressure necessary to deliver the high pressure nitrogen liquid stream from the reflux vessel to the top of the high pressure column.